An example of such a process is described in U.S. Pat. No. 4,388,137. This patent publication discloses a process in which a coating composition is applied to a film before the film and a substrate are pressed together. Next, the coating composition is cured, followed by the film being stripped from the coated substrate. Such a process, in which a coating layer is transferred from a film to a substrate, is sometimes referred to as a film transfer process.
This US publication offers some general information regarding the selection of the coating compositions to be used in the process. Coating compositions without volatile organic compounds, or with only a low level of volatile organic compounds, are not mentioned.
During the drying and curing of coating compositions that contain volatile organic compounds, the main portion of these volatile organic compounds is emitted. Further, when total conversion of the components is not obtained, for instance in the case of UV cure, the uncured low-molecular weight organic molecules can cause environmental problems when the substrate is cut or sanded. Given present day environmental concerns and the corresponding legislation, there is a need for coating compositions without volatile organic compounds or with only a low level of volatile organic compounds.
In U.S. Pat. No. 4,113,894 a process is disclosed in which a substrate is coated with a radiation curable coating composition before a film is placed over the substrate. The substrate and film are irradiated together to cure the coating, after which the film is peeled from the substrate. In the current application, this type of coating process will be referred to as a casting process.
This US publication also does not mention coating compositions with a low level of volatile organic compounds or no volatile organic compounds at all.
In WO 80/01472 a process is disclosed in which a film is coated with a radiation curable coating composition, optionally followed by heating the coated film to evaporate non-polymerisable solvents from the coating. Subsequently, the coated film is applied to a substrate. The coating sandwiched between the film and the substrate is cured by UV radiation, after which the film is removed from the coated substrate. The coating compositions used in this process comprise a high level of organic solvent and/or high level of reactive diluent, i.e. monomers that take part in the curing reaction.
A drawback of this method is that organic solvents may have to be evaporated, while the VOC level is restricted by present day legislation. Such restrictions are expected to be tightened in the near future. Several recycling systems for solvents have been proposed, but such systems and their operation are capital intensive. The use of reactive diluents reduces or eliminates VOC emission, as they are incorporated into the final film. However, they are known for their skin irritant and sensitising properties. Further, these components often have a strong or unpleasant odour and are suspect in view of their toxic properties.
A further problem when coating porous substrates e.g. wood, with compositions comprising reactive diluents is the penetration of the reactive monomers into the pores of the substrate. This is a drawback in particular when the coating is cured by radiation. Since the radiation does not reach these areas, uncured coating material in the pores of the substrate is the result. This can give health, safety, and environmental problems, e.g., when the substrate is cut or sanded. Release of free monomers from porous panels is known to occur even years after the lacquer has been applied.
Well-known acrylic diluents, e.g., tripropylene glycol diacrylate (TPGDA), hexanediol diacrylate (HDDA), acrylated pentaerythritolethoxylate (PPTTA), and hydroxyethyl methacrylate (HEMA) are skin irritants and sensitising and will never react if they are not reached by the UV light.
The use of a process for coating a substrate where the coating composition is sandwiched between the substrate and a radiation permeable film and subsequently cured has several advantages over processes where such a film is absent. A major advantage lies in the fact that the surface configuration on the side of the film facing the coating layer can be imparted to the cured coating. This enables the manufacture of coated substrates with, in principle, any decorative effect. For example, it is possible to make a high gloss coated substrate by using a high gloss film. Low gloss substrates can be manufactured by using low gloss films, which has the advantage that it is not necessary to add a matting agent to the coating composition. It is also possible to manufacture textured coated substrates, for example substrates with a leather- or wood-like structure surface. Since the radiation curable coating is cured in the absence of oxygen, a more durable cured coating with improved (mechanical) properties is obtained.